Weldable and hardenable steel and method of producing same



y 1968 K. G. BAGGSTRM ETAL 3,385,740

WELDABLE AND HARDENABLE STEEL AND METHOD OF PRODUCING SAME Filed June 5, 1967 FIGJ FlG.2

INVENTOR 5 kmu. G'a're Bases-r niim Khan. GlmulDsuNI Plusionl Y Nam. MA

ATTORNEYS United States Patent ()flice 3,385,740 Patented May 28, 1968 8 Claims. (Cl. 148-136) ABSTRACT OF THE DISCLOSURE A weldable, corrosion-resistant, austenitic-martensitic steel, containing about 15 to 40% austenite dispersed in a martensite matrix, is obtained by heating a steel of the composition:

Percent Carbon 0.03-0.25 Silicon 0.10-0.70 Manganese 0.25-2 Chromium 11-14 Nickel 4-8 Molybdenum 0.5-3.5

the remainder being essentially iron, to efiect complete austenization, cooling to room temperature, and tempering at a temperature between about 550 to 650 C.

The present application is a continuation-in-part application based upon application Ser. No. 331,912, filed Dec. 19, 1963, now abandoned.

The present invention relates to a weldable and hardenable steel with a high corrosion resistance in combination with good strength and toughness, and to a method of producing said steel.

United States patent application Ser. No. 404,982, filed Oct. 19, 1964, discloses a method for obtaining a weldable, hardenable and corrosion-resisting steel with good physical properties by means of a special heat treatment of an appropriately chosen alloy.

While this steel has comparatively good corrosion resistance, in certain cases it is desirable to increase this corrosion resistance, without any impairment of the excellent weldability and the other desirable physical properties.

According to the present invention it is now possible to increase the corrosion resistance of the above-mentioned steel considerably, while maintaining its excellent weldability and good physical properties.

The weldable and hardenable steel with a high corrosion resistance in combination with good strength and toughness of the present invention is an alloy of the following composition:

Percent Carbon 0.03-0.25 Silicon 0.10-0.70 Manganese 0.25-2.00 Chrome 11-14 Nickel 4-8 Molybdenum 0.53.5

and the remainder being essentially iron, in which the ratio between the contents of nickel equivalent, calculated as [percent Ni+0.5% Mn] and the chrome equivalent, calculated as [(percent Cr-15% C)+1.5 percent Si+ percent M] is between 0.3 and 0.9. This alloy has been heat-treated through heating so that complete austenitization has been obtained, followed by cooling to room temperature and, finally, tempering at a temperature of between 550 C. and 650 C., whereby a new stable residual austenite is formed in an amount of between and the austenite being dispersed in a martensite matrix.

The alloy can particularly have the following composition:

Percent Carbon 0.03-0.10 0 Silicon 0.20-0.60 Manganese 0.25-2.00 Chrome 11-14 Nickel 4-7 Molybdenum 1.0-2.0

and the remainder being essentially iron.

The term complete austenitization means the conversion of all constituents into austenite when the steel is kept at the elevated temperature. The steel has a complete austenite constitution after this heating as long as it is kept at this temperature, but on cooling this constitution is often altered. However, there are steels which also after cooling have an austenitic constitution, and examples of these are the so-called 18/8 steels containing about 18% Cr and 8% Ni. These austenitic stainless steels are disclosed in Melill (US. Patent No. 3,061,487). In other types of steels the austenitic constitution at the elevated temperature is transformed into martensite on cooling, and steels of this type are also discussed in the patent of Melill.

The steels of Melill are of the semi-austenitic type, which means that they to some extent contain austenite when they are cooled from the complete austenitization heat treatment, and these semi-austenitic steels must be subjected to a specific heat treatment of 4-5 steps to be transformed into the desired final substantially martensitic structure.

In contrast to the above rather complicated heat treatment process, the steel according to this invention is only heated to complete austenitization and air cooled, whereby a substantially martensitic structure is obtained, and afterwards it is reheated to between 550 and 650 C., whereby a stable austenite dispersed in a martensite matrix is produced in an amount of about 15-40%.

In accordance with this invention there are provided steels of specified composition which upon heat treatment and subsequent cooling are transformed into weldable and hardenable steels having high corrosion resistance.

FIGURE 1 is a photomicrograph (X200) of a steel having the composition of a steel of this invention, and treated according to the method of this invention.

FIGURE 2 is a'photomicrograph (X200) of the same steel which has been treated by a different method.

A steel with the following composition:

Percent was melted in a reduced arc furnace and then cast to a 26 inch ingot with a weight of about 6 tons. The ingot was rolled to a mm. thick plate with the size of 4600 x 1500 x 100 mm. This plate was heated to 980 C. for one hour and then cooled in air. From the plate test pieces of about 10 x 10 x 20 mm. were then cut out, and the austenite content is determined by X-ray analysis 3 and photomicrographs prepared. From X-ray measurements it was shown that there is no austenite present in this steel. The photomizrograph taken of this steel is shown in FIGURE 2, and in this photomicrograph only martensitic needles can be identified.

A test piece from the steel as treated above was then subjected to a tempering at 600 C. during hours and afterwards to air-cooling. X-ray measurements of this steel showed it to contain about austenite. The photomicrograph of this steel is shown in FIGURE 1. The microstructure only showed martensitic needles, in spite of the fact that said steel contains 30% austenite, as determined by X-ray analysis. This indicated that the austenite is dispersed in the martensitic matrix and this distribution of the austenite gives the very unexpected 15 and valuable properties to the steels of this invention.

period of time; while, on the other hand, the corrosion in the other four test bars (C-F) was very moderate. After 9 days (the steel of type A was then no longer included) the steel with 1.07% Mo (8) was heavily corroded, while the corrosion of the other four test bars (C-F) was still moderate. However, a tendency towards increasing corrosion as the molbdenum content was increased in these four test bars (C-F) was quite evident. This tendency was still more obvious after 22 days, when the superiority of the steel with 1.45% molybdenum (C) as regards the corrosion resistance became more evident than previously. Thus, a minimum for the corroding speed takes place quite surprisingly at approximately 1.5% molybdenum.

A summary of the physical properties of the six types of steel investigated is given in Table II below.

Table I shows the results of some corrosion tests which included, among others, steel according to the present invention.

TABLE I Molybdenum Weight losses in relative values Type of steel content in after-- percent 3 days 9 days 22 days These corrosion tests have been based upon a steel 40 (type A) with the following composition:

Heating to 980 C. for 1 hour, cooling in air and tempering to 600 C. for 5 hours and, finally, cooling in arr.

The other types of steel used in these tests (B-F) are practically identical to the steel of type A, as regards the composition and the heat-treatment, but with the exception, however, that the molybdenum content was varied in the way indicated in Table I. For the actual corrosion test, an oxygensaturated 5% salt solution was used, to which had been added 21 g. of citric acid per litre. The liquid, which had a pH of 1.9, was allowed to circulate in flasks with test bars made of the different ypes of steel, at a temperature of +50 C. At certain time intervals, the weight losses of the different test bars were determined. The loss of weight which took place as a consequence of the corrosion in steel of type A after 3 days has been given the rating 100, and the weight losses of the other steels have been indicated in relation to this value.

In addition to the above-mentioned corrosion of the steel (A), which was practically free from molybdenum,

after 3 days, the steel with 1.07% molybdenum (B) also The physical properties were determined by entirely conventional methods.

From Table II, it will be noted that an increase of the molybdenum content results in, among other things, an increase of the ultimate tensile strength, but a decrease of the elongation, and reduction of area and impact strength. These tendencies were particularly prominent in the test bar with 3.50% molybdenum (F) and apply especially to the impact strength.

All the types of steel tested showed good weldability, and this fact is supported by the comparatively high contents of residual austenite, which are indicated in Table II. The residual austenite was determined by X-ray analysis.

.From Tables I and II it will be noted that, in order to obtain good corrosion resistance, there should also be at least 0.5% molybdenum in the steel, while in order that the impact strength should not decrease too much, the molybdenum content should be less than 3.5%.

The tests which have been set forth were made with test bars taken from blanks which had been forged, but corresponding tests made with cast steel have given similar results.

We claim:

1. A heat treated, weldable. corrosion-resistant austenitic-martensitic steel, containing about 15 to 40% austenite dispersed in a martensite martix, which consists essentially of:

Percent Carbon 0.03-0.25 Silicon 0.10-0.70 Manganese 0.25-2 Chromium 11-14 Nickel 4-8 Molybdenum 0.5-3.5

the balance essentially iron, wherein the ratio between the nickel equivalent, calculated as (percent Ni+0.5% Mn), and the chromium equivalent, calculated as [(percent Cr15% C)+1.5 percent Si+ percent M0] is in the range of about 0.3 to 0.9.

2. A steel in accordance with claim 1, wherein the molybdenum content is 102.0%.

3. A steel in accordance with claim 2, wherein the silicon content is 0.2 to 0.6%.

4. A steel in accordance with claim 1, wherein the carbon content is about 0.1% and the nickel content is about 7%.

5. Method of producing a steel as defined in claim 1 which comprises heating a steel of the composition as 5 6 defined in claim 1 to effect complete austenizing thereof, References Cited cooling to room temperature, and tempering said steel UNITED STATES PATENTS at a temperature between about 550 to 650 C., thereby producing a stable austenite therein in an amount of 1,404,907 1/1922 Strauss 148136 X about15t040% 5 2,802,755 8/1957 Bloom 7s 12s 3,061,487 10/1962 Melill et al. 148143 6. Method according to claim 5, wherein the molybdenum content of the steel is 1.02.0%.

7. Method according to claim 6, wherein the silicon OTHER REFERENCES content of the steel is 0.2 to 0.6%. Bullens: Steel and Its Heat Treatment, vol. 1, John 8. Method according/co claim 5, wherein the carbon 10 Wiley & Sons Inc., New York, relied on pp. 453-456. content of the steel is about 0.1% and the nickel content is about 7%. CHARLES N. LOVELL, Primmy Examiner. 

